Heating and ventilating, or air conditioning, installations for the cabin of a motor vehicle usually include a control device for controlling airstreams, comprising a movable obturating member in the form of a pivoting or sliding valve member arranged in a duct for the corresponding airstream. The pivoting or sliding movement of these valve members, and the presence of actuating means for the latter, involves quite a large component size in the direction of the airstream and/or transversely to the latter, which makes such devices difficult to fit in the restricted space which is available below the fascia of the vehicle.